Singaling system



'- BAND F PASS Feb. 9, 1943.

R. E. RESSLER SIGNALING SYSTEM Filed March 31, 1942 FIG.

' E BAND PASS FILTER AND 1-, EQUAL/25R FREQUENCY DETECTOR AMPLITUDE 05mTOR 2 Sheets-Sheet 1 CONTROL LEAD INVENTOR R. E. 55L ER 8 ATTORNEY Feb.9, 1943. I RESSLER 2,310,749

SIGNALING SYSTEM Filed March 51, 1942 '2 Sheets-Sheet 2 F/G. 4 POLAR/ZED9 5 5 SR"4 T a0 7 3% J 9/ a/ a2 BPF- 4 M H J 86 l BAND I I I PASS c |I1|I|I|l POLARIZED 98/ 1:4 155 4 92 L :auu- 73 77/ /Z n E 76 92 =K72 IINVENTOR z 74 75 R. ERESSL ER L- I 08mm ATTORNEY Patented Feb. 9, 1943UNITED STATES PATENT orrloe SIGNALING SYSTEM Ralph E. Ressler,Morristown, N. J., assignor to Bell Telephone Laboratories;Incorporated, New York, N. Y., a corporation of New York ApplicationMarch 31, 1942, Serial No. 436,979 21 Claims. (Cl. 179-84) Thisinvention relates to signaling systems and particularly to systemsemploying voice frequency currents for transmission of signals.

Objects of the invention are an increase in the reliability of signalingsystems employing voice frequency currents for signaling and theprevention of false signal operations in response to voice frequencycurrents other than signaling currents.

In systems in which alternating currents of voice frequency are employedfor signaling over lines which are also used for the transmission ofspeech, various arrangements have been employed to guard against signaloperations in response to voice currents or other interfering currentsincluding the frequencies used for signal transmission. This inventionis a signaling system comprising a signal receiving circuit arranged tooperatively respond to signals consisting of frequency modulatedcurrents of constant amplitude and arranged to prevent signal operationin response to voice currents of varying amplitude, voice currents beingsubjected to sufiicient amplitude modulation to effect the disabling ofthe signal channel.

A feature of the invention is a signal receiver comprising a signalingchannel and a signal relay operatively responsive to signaling currentof a particular frequency characteristic and constant amplitude andarranged to prevent operation of the signal relay responsive to currentof like frequency characteristic but varying in amplitude.

Another feature of the invention is a signal receiver adapted to respondto signals consisting of current of constant amplitude varying betweentwo frequencies at a constant rate, the variation in frequency with timebeing preferably sinusoidal in character; operation of the receiver inresponse to voice currents or other interfering currents which includethe signaling frequencies being prevented by the variation in amplitudeto which the voice currents or other interfering currents are subjected.

Another feature of the invention is a signal receiver arranged torespond to voice frequency signaling current of constant amplitudevarying between two predetermined frequencies at a constant rate; andarranged to receive other signals consisting of current of constantamplitude varying between two other predetermined frequencies at thesame rate at which the frequency of the current of a signal of thefirst-mentioned character is varied.

A further feature of the invention is a signal receiver arranged torespond to signals consisting of current of constant amplitude varyingbetween two frequencies at one rate and arranged tov respond to othersignals consisting of current varying in frequency between the same twofrequencies but varying at a different rate.

The invention will be better understood by considering specific signalreceivers in which the invention and its various features are embodied.Three such signal receivers are shown in Figs. 1, 2 and 3 of thedrawings which form a part of this specification. The invention is notlimited in its application to the receivers shown in the drawings, butis generally applicable to signaling circuits for responding to voicefrequency signals.

Referring to the drawings:

Fig. 1 shows in schematic form the general arrangement of a signalreceiver SRI embodying this invention, consisting of a frequencydetector and an amplitude detector connected by a filter and equalizerto a line Ll Fig. 2. shows a line L-2 connected by a bandpass filter andequalizer BPF-2 to a signal receiver SR-Z. comprising a frequencydetector and an amplitude detector;

Fig. 3 shows a signal receiver SR-3 of like character to that shown inFig. 2 except that it is arranged to respond to voice frequency signalsof two different characters; and

Fig. 4 shows another signal receiver SR4 arranged to. respond to voicefrequency signals of two different characters.

In a, system arranged in accordance with applicants invention, thesignals impressed upon the transmission line are of constant amplitudebut vary in frequency between a value FI and F2, the variation being ata constant rate p. At the receiving end the signal passes from the linethrough a band-pass filter which passes a band at least as wide as twicethe rate at which the frequency is varied and as much wider as isnecessary, depending upon the difference between the twofrequenciesbetween which the frequency varies. The associatedv equalizer isadjusted for constant amplitude of the tone at the filter output.terminal to compensate for frequency twist in the line and equipment.The output from the filter undergoes a conversion from frequency toamplitude modulation in the frequency detector, the resultant rectifiedwave having a fundamental frequency equal to the rate at which thefrequency is Varied or twice the rate at which the frequency is varieddepending upon the manner of effecting the conversion. This wave is thenpassed through a tuned circuit or filter, rectified and v pplied tothewinding of a signal receiving relay. Amplitude modulated voice currentsor other interfering currents of varying amplitude which pass throughthe band-pass filter cause the amplitude detector to function, theresultant output being employed to disable the signaling channel andthus prevent false operation in response to voice or other interferingcurrents.

Referring now to the signal receiver SR-Z shown in Fig. 2, the incomingsignaling current, varying at a rate p between frequencies Fl and F2, istransmitted through band-pass filter BPF--2, transformer T-2 andamplifier A2, to the input winding of transformer Ill. The equalizer isadjusted to produce constant signal amplitude between frequencies Fl andF2 in the output windings of transformer Ill. The incoming signal isthus impressed through the upper right winding of transformer l on thegrid of amplifier tube l9. An antiresonant circuit, comprising thecondenser 20 and inductor 2| and tuned to the mean signaling frequency,is connected across the primary winding of transformer 22 in theanode-cathode circuit of tube IS, an alternating current voltage whichis amplitude modulated at a frequency 2p, equal to twice the rate ofvariation between frequencies Fl and F2, being thereby applied throughtransformer 22 to the grid of detector tube 23. The anode-cathodecircuit of tube 23 includes a network comprising inductance 24 andcondenser 25, tuned to the frequency 2p so that a voltage of thisfrequency is applied to the full wave rectifier 26 to operativelyenergize the upper winding of signal relay 21; and, if there is nocurrent of varying amplitude being received, relay 21 responds to thesignaling current. Relay 2'! disconnects ground from conductor 28 andconnects ground to conductor 29 to effect any desired signal operation.I

The lower right winding of transformer I0 is connected across therectifying tube l l; and variations in the rectified currents areeffective to impress an alternating potential across condenser l2 andacross inductance l3 and the left winding of transformer l4. Thecondenser l2 and inductance l3 constitute a low-pass filter which has acut-01f frequency between 2p and 2Fl. If the amplitude of the outputwave in the anodecathode circuit of tube A2 isconstant, no voltage willappear across the secondary winding of transformer l4 after the initialbuild-up surge; but any amplitude modulation, at frequencies below thecut-off frequency produces an output voltage across the right-handwinding of transformer l4 which is rectified in the tube l5 and appearsas a direct current potential across resistor l6 and condenser l'l. Anydirect current potential thus built up across the resistor l6 andcondenser l'l constitutes a negative grid bias which is impressed,through conductor l8 and the upper right winding of transformer l0, onthe grid of tube l9; and this grid bias tendsto disable the signalchannel and thus prevent false operation of the signal relay 2! inresponse to voice currents or other interference current containingamplitude modulated components of signaling frequencies.

The basic method of discriminating between signals and voice currents,illustrated in the signal receiver SR-2 tended to include means forreceiving and reproducing another signal of a different character. Thesignal receiver shown in Fig. 3 includes a signal relay 52 forresponding to signals consisting of current of constant amplitudevarying at of Fig. 2, can be readily exwinding of transformer 30 a ratep between frequencies Fl and F2 and includes a signal relay 55 forresponding to signals consisting of signaling current of constantamplitude varying in frequency between frequencies F3 and F6 at the samerate 10. The receiver includes a dual-diode triode 40 used as anamplifier and full wave rectifier and a dual-diode triode 48 used'as adetector and full wave rectifier. Current of signaling frequenciesincoming over line L-3 is applied through a band-pass filter andequalizer EFF-3, transformer T-3, and amplifier A3 to the primarywinding of transformer 30, the voltage induced in the upper right beingapplied to the grid of tube 40. The anode-cathode circuit of tube 40includes two tuned networks. One of these networks comprises inductance4| and condenser 43 and it is tuned to the mean frequency betweenfrequencies Fl and F2. The other of these networks comprises inductance42 and condenser 44; and it is tuned to the mean frequency betweenfrequencies F3 and F4. When the current in line L-3 includes signalingcurrent varying in frequency at a rate p between frequencies Fl and F2,a voltage of the mean frequency between Fl and F2 is impressed acrossrectifier 45, the output current of the rectifier being efiective toenergize the upper winding of relay 55 in the operating direction; and,when the current in line L-3 includes signaling current varying infrequency at a rate p between frequencies F3 and F4, a voltage of meanfrequency between F3 and F4 is impressed across rectifier 46, the outputcurrent of the rectifier being efiective to energize the middle windingof relay 55 in the releasing direction. Relays 52 and 55 are polarized,the lower winding of each being a biasing winding. With both of relays54 and 55 normal, the current in the biasing winding of relay 55 is inthe non-operating direction, the bias being insufficient to prevent theoperation of relay 55 by current through its upper energizing winding.With both of relays 54 and 55 operated, the current through the biasingwinding of relay 55 is in the operating direction and the bias issulficient to hold relay 55 operated without current in the upperwinding but not sufficient to prevent release of relay 55 in response toenergization of the middle winding upon receipt of dial impulse signalsover line L-3 as hereinafter described. With relay 55 operated and relay54 released, the current in the biasing winding of relay 55 is in theoperating direction and the bias is suflicient to hold relay 55operated, the energization of the middle winding in the non-operatedirection being ineffective to cause the release of relay 55 as long asrelay 54 remains normal. The anode-cathode circuit of tube 40 furtherincludes the left winding of transformer 41 connected in parallel withinductances M and 42; and, whenever the current in line L3 includessignaling current of either character, a voltage of like frequencycharacteristic is impressed through transformer 41 on the grid ofdetector tube 48. The anode-cathode circuit of tube 48 includes anetwork comprising inductance 49 and condenser 50 tuned to frequency2;); so that, whenever signaling current of either of the twocharacteristics is present in line L3, a voltage of frequency 2;) isimpressed across rectifier 5| to operatively energize'the upper windingof relay 52.

Upon transmission of a seizure signal consisting of current of constantamplitude varying in frequency at arate'p between frequencies Fl and F2over line L--3, the upper windings of relays 52 and 55 are operativelyenergized. The operation of relay 52 closes a circuitfor operating relay54 and disconnects ground from conductor 53 to effect any desiredswitching operation during the seizure signal. The operation of relay 55disconnects ground from conductor 55 to reverse the direction of thecurrent in the biasing winding of relay 55 and connect ground toconductor 51 to prepare for the reception of dial impulse signals. Theoperation of relay 54 connects ground to conductor 58, reducing thebiasing current in the lower winding of relay 55. When the seizuresignal ends, relays 52 and 54 release in succession, the release ofrelay 54 being effective to disconnect ground from conductor 58 andthereby increase the current in the biasing winding of relay 55 to givethe maximum bias in the operating direction. When a calling operator orcalling subscriber connected to the distant end of line L3 dials a digitof a called route or'call'ed subscribers number, current of the samecharacter as the seizure signal is transmitted over line L-3 precedingthe first dial impulse signal, during each interval between impulsesignals, and at the end of the last impulse-signal in the traincorresponding to the digit dialed. The signaling current preceding thefirst impulse signal effects the reoperation of relay 52; and theresulting reoperation of relay 54 decreases the current in the biasingwinding of relay 55 to render this relay responsive to the succeedingdial impulse. When the current of the first character ceases and thefirst impulse signal is received,

relay 55 is released due tothe energization of its 1 middle Winding, butrelay 52 is held operated since the impulse signal continues theenergizationof the operating winding of relay 52. Each dial impulsesignal isthus effective to release relay 55, thereby disconnectingground from condoctor 5! once for each dial impulse signal received.After the last impulse of the series, the current of the same characteras the seizure signal effects the reoperation of relay 55; and, whenthis current ceases, relay 52 releases but relay 55 remains operated.

When a disconnect signal, consisting of current of the same character asthe seizure signal followed by a long impulseof current of the samecharacter as the dial impulse signal, is received, relays 52 and 54 areoperated and relay 55 is released to effect the release of any switchesthrough which the connection extends. When the disconnect signal ends,relay 55 does not reoperate and relays 52 and 54 release.

If now voice currents or other interfering currents including the samefrequencies as the signaling currents but varying in amplitude arereceived over line L3, the voltage induced in the lower winding oftransformer 30 is rectified by the cathode and rectifier plates of tube40, and the rectified voltage is impressed across condenser 32. The leftwinding of transformer 34, inductance 33 and condenser 32 constitute alowpass filter which passes current of frequencies below the secondharmonic of the lowest important side-band frequency present in theincoming signal wave. Voice or other interference currents of varyingamplitude passed by filter BPF3 are thus effective to. induce voltagesin the right winding of transformer 34 which are rectified by thecathode and rectifier plates of tube 48 to create a direct currentpotential across the resistor 35 and condenser 36 which constitutes anegative bias in the grid-cathode circuit of tube 40. Thus voice orother interference currents create a negative bias whichblocks amplifier40 to prevent. false operation of the signaling relay. It' is to be.noted, however, that each time relay 54 is operated in response tosignaling current of either character in line L-3, relay 54 connectsground to conductor 39 to short-circuit resistor 35 and condenser 36,.thereby preventing the. amplitude detector from disabling the signalingchannel so that amplitude surges occurring during the transmission ofsignals after the operation of relay 54' will be ineffective to bias thegrid of tube.

Referring now to Fig. 4 the signal receiver SR4 is arranged to respondto two signals of constant amplitude but varying frequency, the firstsignal consisting of current varying at a rate p l between frequenciesFl and F2 and the second signal consisting of current varying at a ratep2 between the same frequencies Fl and F2. This signal receiver has theadvantage of not requiring as wide a frequency band for use in signalingas does the receiver SR-3 of Fig. 3. Relays 9i and 95 are similar torelays 52 and 55 of receiver SR3 except that relay 9l has an additionalwinding, the upper winding of relay 9| being connected in series withthe 95 and the middle winding of relay 9| being connected in series withthe lower winding of relay 95. Current of signaling frequencies in lineL4 is transmitted through the band-pass filter and equalizer BPF4,transformer T-4 and amplifier A4 to the input winding of transformer 10,the voltages induced in the upper right winding of transformer 10 beingapplied to the grid of tube 85. The anode-cathode circuit of tubeincludes a network, comprising condenser 8| and inductance 82 tuned tothe mean signaling frequency, connected in parallel with the lefthand.winding of transformer 83, voltages of this frequency being transmittedthrough transformer 83 to the grid of detector tube 84. The anodecathodecircuit of tube 84 includes two tuned networks, the network comprisinginductance 85 and condenser 81 being tuned to frequency pl at which thefrequency of the signaling current of the first character is varied andthe network including inductance 86 and condenser 88 being tuned tofrequency p2 at which signaling current of the second character isvaried. Thus voltages of frequency pl are impressed across rectifier 89in response to signaling current of the first character to energize themiddle winding of relay 95 and upper winding of relay 9 l in series; andvoltages of frequency 202 are impressed across the rectifier 90 inresponse to signaling current of the second character to energize thelower winding of relay in series with the middle winding of relay 9|;and relay 9 l is operatively energized by the one or the other of itsoperating windings in response to signaling currents of either characterin line L-4. The energization of the middle Winding of relay 95 inresponse to signaling current of the first character in line L-4 iseffective to operate relay 95; whereas the energization of the lowerwinding of relay 95 in response to signaling current of the secondcharacter is effective to cause the release of relay 95, as hereinafterfurther described. If the current in line L--4 includes voice currentsor other currents varying in amplitude, the voltages thus induced in thelower right winding of transformer 10 are rectified in the cathode andrectifier plate circuits of tube 80 and impressed across a networkcomprising condenser 12, inductance l3 and the middle winding of relayvleft-hand winding of transformer 14. This net work constitutes alow-pass filter having a cutoff frequency below the second harmonic ofthe lowest important side-band frequency present in the incoming signalwave; and voltages below the cut-off frequency are thus induced in therighthand winding of transformer 14 and rectified in the cathode andrectifier plate circuits of tube 84 to create a direct current drop inpotential across resistor 15 and condenser I6. This direct currentdifference in potential constitutes a negative grid bias in thecathode-grid circuit of tube 80 so as to block the signaling channelwhenever voice current or other current of varying amplitude occurs inline L- Upon transmission of a seizure signal over line L-4, there beingno interfering currents to block the signaling channel, relays 9| and 95are operated by the energization of their upper and middle windingsrespectively. The operation of relay 95 disconnects ground fromconductors 96, thereby reversing the direction of the current in theupper, biasing winding of relay 95 to bias this relay in the operatingdirection; and connects ground to conductor 91 to prepare the switchingapparatus for response to dial impulses. The operation of relay 9|closes a circuit for operating relay 92. Relay 92 disconnects groundfrom conductor 93 to perform any desired switching operation such asopening the talking transmission circuit, connects ground to conductor99 to reduce the current in the biasing winding of relay 95, andconnects ground to conductor 19 to disable the blocking channel of thesignal receiver. When the seizure signal ceases, relays 9| and 92release in succession, but relay 95 is held operated by the current inthe biasing winding. When the calling operator or calling subscriber atthe distant end of line L4 begins to dial the called route or calledsubscribers number, signaling current varying in frequency between Fland F2 at a rate pl is transmitted over line L-& preceding the firstdial impulse to effect the reoperation of relays 9| and 92; and currentof like character is transmitted during the interval between impulsesand at the end of the last impulse signal of the train corresponding tothe digit dialed. Each dial impulse signal, consisting of current ofconstant amplitude but varying in frequency at a rate p2 between Fl andF2, effects the energization of the middle winding of relay 9| to holdthis relay operated and the energization of the lower winding of relay95 to cause the release of relay 95. When each dial impulse ends, thesignaling current varying in frequency at a rate pl energizes the upperwinding of relay 9| to hold this relay operated and energizes the middlewinding of relay 95 to cause the reoperation of relay 95. Thus each dialimpulse causes the release of relay 95 to transmit an impulse overconductor 91 to any switching apparatus to be controlled by the dialimpulses. After the last dial impulse of the train, relay 95 isreoperated by the-current varying in frequency at a rate pl to againclose the circuit over conductor 91 to hold the switching apparatus.When a disconnect signal consisting of current varying in frequency at arate pl followed by current varying in frequency at a rate p2 isreceived, relays 9| and 92 reoperate and relay 95 releases to open thecircuit over conductor 91 and effect the release of the switchingapparatus. When the disconnect signal ceases, relays 9| and 92 arereleased in succession and since the disconnect signal is ineffective toreenergize the middle winding of relay 95, this. relay remains in itsreleased condition.

Various modifications may be made in the signal receiver shown in Figs.2, 3 and 4 within the scope of the invention.

What is claimed is:

1. A signal receiver for responding to signals consisting of frequencymodulated signaling current of constant amplitude without responding toamplitude modulated current of like frequencies, said receivercomprising a frequencydetector, a signal relay operatively controlled bysaid detector in response to said signals, and means including anamplitude detector for blocking the operation of said frequency detectorto prevent operation of said relay responsive to current varying inamplitude.

2. A signal receiver for connection to a line adapted to transmit voicecurrents, said signal receiver adapted to respond to signals consistingof signaling current of a particular frequency characteristic andconstant amplitude without responding to voice currents or otherinterference currents of like frequency but varying amplitude, saidreceiver comprising a signaling channel including a signal relayoperatively connected for response to signaling current and a blockingchannel responsive only to current of varying amplitude for disablingthe operation of said signaling channel.

3. A signal receiver comprising input terminals, a signal relay, meansincluding a frequency detector connecting said relay to said terminalsfor operation in response to the impression of a signaling voltagevarying in frequency at a predetermined rate between predeterminedlimits, and means including an amplitude detectoreffective in responseto the impression of voltages of varying amplitude across said terminalsto render said frequency detector inoperative.

4. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, and means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay.

5. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay, and means including means for detecting variations in theamplitude of the current passed by said filter for preventing theoperation of said relay.

6. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminal to the input sideof said amplifier, said filter adapted to pass alternating currentwithin two predetermined limiting frequencies, means connecting theoutput side of the first-mentioned amplifier to the input side of saidother amplifier, a detector, a transformer connecting the anode-cathodecircuit of said other amplifier to the input side of said detector, anantiresonant network tuned to the mean frequency of said limitingfrequencies, said network connected in parallel with the primary windingof said transformer to impress on said detector a voltage modulated inamplitude at a frequency equal to twice the rate at which the frequencyof the voltage impressed on said terminals varies between said limitingfrequencies, a signal relay, and means in the anodecathode circuit ofsaid detector including means tuned to resonance at a predeterminedfrequency for operatively energizing said relay when the voltageimpressed on said terminals varies in frequency between said limitingfrequencies at a rate equal to the last-mentioned predeterminedfrequency.

7. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay, and means including means for detecting variations in theamplitude of the current passed by said filter for preventing theoperation of said relay.

8. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a Winding of saidrelay, and means including means for detecting variations in theamplitude of the current passed by said filter for controlling theoperation of said amplifier to prevent the operation of said relay.

9. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay, and means including means for detecting variations in theamplitude of the current passed by said filter for impressing a negativebias on the control grid of said amplifier.

10. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay, and means including means for detecting variations in theamplitude of the current passed by said filter for impressing a negativebias on the control grid of said amplifier, sufficient to prevent theoperation of said relay.

11. A signalreceiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay, biasing means connected in the grid-cathode circuit of saidamplifier, and means including means for detecting variations in theamplitude of the current passed by said filter for building up a biasingpotential across said biasing means.

12. A signal receiver comprising input terminals, an amplifier, meansincluding a band-pass filter connecting said terminals to the input sideof said amplifier, said filter tuned to pass alternating current withinpredetermined limiting frequencies, a detector, means including a tunednetwork antiresonant at the mean frequency between said limitingfrequencies connecting the output of said amplifier to the input of saiddetector, a signal relay, means including a network resonant at apredetermined frequency connecting said detector to a winding of saidrelay, a grid condenser and grid leak resistor connected in thegrid-cathode circuit of said detector, and means including rectifyingmeans and a filter adapted to pass current of frequencies below acut-off frequency lying between twice the lastmentioned predeterminedfrequency and twice the lowest of said predetermined limitingfrequencies connecting said grid condenser and resistor to thefirst-mentioned filter, whereby the impression of a voltage which variesin amplitude across said terminals effects the building up of a negativebiasing potential across said grid condenser and resistor to prevent theoperation of said relay.

13. A signal receiver comprising input terminals, a signal relay foroperation responsive to signals consisting of currentof constantamplitude varying in frequencyat a predetermined rate betweentwopredetermined limiting frequencies, and means operatively connectinga winding of said relay to said filter, said means including a band-passfilter adapted to pass current within said limiting frequencies,including a network antiresonant at a mean frequency between saidlimiting frequencies, and including a detector tuned to a frequencyequal to twice said predetermined rate, and means including means fordetecting variations in the amplitude of the current passed by saidfilter for preventing the operation of said relay.

14. A signal receiver comprising input terminals, a band-pass filterconnected to said terminals adapted to pass current of frequenciesbetween two predetermined frequencies PI and F2 and to pass current offrequencies between two different predetermined frequencies F3 and F4, asignal means connected to said filter and oper- 6 ative in response tosignaling current varying in frequency at a predetermined rate 10between frequencies Fl and F2, and a signal means connected to saidfilter and operative in response to signaling current varying infrequency at said rate 1) between said frequencies F3 and F4.

15. A signal receiver comprising input terminals, a band-pass filterconnected to said terminals adapted to pass current of frequenciesbetween two predetermined frequencies Fl and F2 and to pass current offrequencies between two different predetermined frequencies F3 and F4, asignal means connected to said filter and operative in response tosignaling current varying in frequency at a predetermined rate p betweenfrequencies Fl and F2, a signal means connected to said filter andoperative in response to signalin current varying in frequency at apredetermined rate 1) between said frequencies F3 and F4, and meansconnected to said filter responsive to current of varying amplitude forpreventing the operation of either of said signal means.

16. A signal receiver comprising input terminals, a signal relay, aband-pass filter connected to said terminals and adapted to pass currentof frequencies between two predetermined frequencies Fl and F2 andadapted to pass current between two difierent predetermined frequenciesF3 and F4, means including a network tuned to the mean frequency betweenFl and F2 for operating said relay in response to signaling currentvarying in frequency at a predetermined rate 10 between Fl and F2, andmeans including a network tuned to the mean frequency between F3 and F4for releasing said relay in response to signaling current varying infrequency at said rate p between F3 and F4.

17. A signal receiver comprising input terminals, a signal relay, aband-pass filter connected to said terminals and adapted to pass currentof frequencies between two predetermined frequencies Fl and F2 andadapted to pass current between two different predetermined frequenciesF3 and F4, means including a network tuned to the mean frequency betweenFl and F2 for operating said relay in response to signaling currentvarying in frequency at a predetermined rate p between FI and F2, meansincluding a network tuned to the mean frequency between F3 and F4 forreleasing said relay in'response to signaling current varying infrequency at said rate 1) between F3 and F4, and means connected to saidfilter for detecting variations in amplitude of current passed by saidfilter for preventing operative response by said relay.

18. A signal receiver comprising input terminals, two signal relays, aband-pass filter connected to said terminals adapted to pass current offrequencies between two predetermined .frequencies Fl and F2 and adaptedto pass current of frequencies between two other predeterminedfrequencies F3 and F4, an amplifier connected to said filter, meansincluding a net work tuned to the mean frequency between FI and F2operatively connecting one of said relays to said filter for response tosignaling current varying in frequency between Fl and F2, and meansincluding a network tuned to the mean frequency between F3 and F4operatively connecting both of said relays to said filter for responseto signaling current varying in frequency between F3 and F4.

19. A signal receiver comprising input terminals, two signal relays, aband-pass filter connected to said terminals adapted to pass current offrequencies between two predetermined frequencies Fl and F2 and adaptedto pass current of frequencies between two other predeterminedfrequencies F3 and F4, an amplifier connected to said filter, meansincluding a network tuned to the mean frequency between Fl and F2operatively connecting one of said relays to said filter for response tosignaling current varying in frequency between Fl and F2, meansincluding a network tuned to the mean frequency between F3 and F4operatively connecting both of said relays to said filter for responseto signaling current varying in frequency between F3 and F4, and meansconnected to said filter and including means for detecting variations inamplitude of current passed by said filter for preventing the operativeresponse of said relays.

20. A signal receiver comprising input terminals, two signal relays, aband-pass filter connected to said terminals and adapted to pass currentbetween two predetermined frequencies FI and F2, means connected to saidfilter and responsive to signaling current varying in frequency betweenFl and F2 at a predetermined rate for operating one of said relays, andmeans connected to said filter and responsive to signaling currentvarying in frequency between Fl and F2 at a different predetermined ratefor operating the other of said relays.

21. A signal receiver comprising input terminals, two signal relays, aband-pass filter connected to said terminals and adapted to pass currentbetween two predetermined frequencies Fl and F2, means connected to saidfilter and responsive to signaling current varying in frequency betweenFl. and F2 at a predetermined rate for operating one of said relays,means connected to said filter and responsive to signaling currentvarying in frequency between Fl and F2 at a different predetermined ratefor operating the other of said relays, and means connected to saidfilter and responsive to current varying in amplitude for preventing theoperation of either of said relays.

RALPH E. RESSLER.

